1. Field of the Invention
The present invention relates to a plasma accelerator, and more particularly, to a plasma accelerator which winds driving coils in opposite directions and connects the driving coils with one another, thereby reducing a mutual inductance between the driving coils and accurately adjusting levels and phase differences of currents applied to the driving coils, and also simplifies a driving circuit.
2. Description of the Related Art
A plasma accelerator accelerates flux of plasma generated or existing in a specific space using electric energy and magnetic energy and is called an ‘electro-magnetic accelerator’ (EMA).
The plasma accelerator has been developed for a rocket engine for space travel and is recently used in etching a wafer in the process of manufacturing a semiconductor.
FIG. 1 is a cross section view illustrating a conventional plasma accelerator.
As shown in FIG. 1, the conventional plasma accelerator is in the shape of a cylinder and has a top coil T wound around an entrance, which is an upper portion of the plasma accelerator. A first coil S1, a second coil S2, a third coil S3, a fourth coil S4, and a fifth coil S5 are wound around from the entrance to an exit in sequence. The coils T, S1, S2, S3, S4 and S5 are not connected to one another and are wound independently.
Accordingly, individual currents are applied to the coils T, S1, S2, S3, S4 and S5. Plasma is generated in the plasma accelerator by applying RF currents to the coils T, S1, S2, S3, S4 and S5. The currents flowing through the coils T, S1, S2, S3, S3 and S5 generate a magnetic field in the plasma accelerator.
The magnetic field generated in the plasma accelerator by the currents flowing through the coils T, S1, S2, S3, S4, and S5 induces a second current according to Maxwell induction equation, and the second current converts gas in the plasma accelerator into plasma.
According to a plasma acceleration method of the conventional plasma accelerator, a current of 40A is applied to the first coil S1, the third coil S3 and the fifth coil S5, and a current having a phase difference of 90° is applied to the top coil T, the second coil S2 and the fourth coil S4. (A refers to Amperes.) The magnetic field is generated by the currents and accelerates the plasma toward the exit.
FIG. 2 is a graph illustrating a magnitude of magnetic field generated in the plasma accelerator of FIG. 1.
In FIG. 2, the horizontal axis indicates a vertical distance from the entrance of the plasma accelerator to the exit, and the vertical axis indicates a magnitude of magnetic field generated in the plasma accelerator for a specific time.
The circles indicate the second current that is induced by the magnetic field generated in the plasma accelerator, and the arrow indicates a direction in which the plasma is accelerated.
A magnetic pressure distribution is B2/2μ. Herein, B denotes a magnetic flux density and μ denotes a permeability. At the first time, the current of 40A, which is the maximum AC current (alternating current), is applied to the top coil T, the second coil S2, and the fourth coil S4, and the current having a phase difference of 90° is applied to the first coil S1 and the third coil S3. That is, the current of 0A is applied to the first coil S1 and the second coil S3. Then, a magnetic pressure is distributed in the plasma accelerator.
Next, a current of 30A is applied to the top coil T, the second coil S2 and the fourth coil S4, and a current having a phase of 90° is applied to the first coil S1 and the third coil S3. Then, the magnetic pressure moves toward the exit.
Next, a current of 10A is applied to the top coil T, the second coil S2 and the fourth coil S4, and a current having a phase difference of 90° is applied to the first coil S1 and the third coil S3. Then, the magnetic pressure further moves toward the exit.
Next, a current of 0A is applied to the top coil T, the second coil S2 and the fourth coil S4, and a current having a phase difference of 90° is applied to the first coil S3 and the third coils S3. Then, the magnetic pressure further moves toward the exit.
As described above, the magnetic pressure distribution progressively changes toward the exit as the time elapses. Due to the change in the magnetic pressure distribution, the plasma moves toward the exit. In order to be moved by the magnetic pressure, the plasma has to be ahead of a magnetic pressure pulse. This is because the gradient of the magnetic pressure creates a magnetic pressure force and has the plasma flow toward the exit.
If the magnetic pressure force is so weak that the plasma is slow, the plasma does not follow the change of the magnetic pressure distribution. Therefore, the plasma accelerator cannot accelerate the plasma. In this case, reducing a gap between coils and lowering a frequency of driving current makes the motion of the magnetic pressure distribution slow.
The following table 1 is a matrix that shows a self-inductance and a mutual inductance of the coils in rows and columns. Since the distribution of the inductance values is diagonally symmetrical, the repeated inductance values are omitted. The inductance value is expressed in the unit of μH (microhenries).
TABLE 1TS1S2S3S4S5T0.3930.0920.0230.0070.0010.000S11.0600.2190.0600.0190.004S21.0600.2190.0600.019S31.0600.2190.060S41.0600.219S51.060
A total of 6 coils T1, S1, S2, S3, S4, S5 are applied with individual currents from 6 divided RF generators.
A magnetic energy W stored in the coil is calculated according to the following equation 1:
  W  =                              L          1                ⁢                  I          1          2                    2        +                            L          2                ⁢                  I          2          2                    2        +                  M        12            ⁢              I        1            ⁢              I        2            wherein I1 denotes a current flowing through the first coil, I2 denotes a current flowing through the second coil, L1 denotes a self-inductance of the first coil, L2 denotes a self-inductance of the second coil, and M12 denotes a mutual inductance
If the current I1 is set to 0 and the current I2 is set to 1, a magnetic energy W is calculated and thus L1, L2, and M12 can be obtained. Accordingly, each element of the matrix of the table 1 can be calculated.
However, the mutual inductances of the coils T, S1, S2, S3, S4 and S5 have considerable magnitudes. Due to the mutual inductances, the current is likely to be unstably applied to the coils. The unstable level of current and the unstable phase difference cause an unstable operation of the plasma accelerator.